N-phosphonomethylglycine, commonly referred to as glyphosate, is an important agronomic chemical. Glyphosate inhibits the enzyme that converts phosphoenolpyruvic acid (PEP) and 3-phosphoshikimic acid (S3P) to 5-enolpyruvyl-3-phosphoshikimic acid. Inhibition of this enzyme (5-enolpyruvylshikimate-3-phosphate synthase; referred to herein as “EPSPS”) kills plant cells by shutting down the shikimate pathway, thereby inhibiting aromatic acid biosynthesis.
Since glyphosate-class herbicides inhibit aromatic amino acid biosynthesis, they not only kill plant cells, but are also toxic to bacterial cells. Glyphosate inhibits many bacterial EPSP synthases, and thus is toxic to these bacteria. However, certain bacterial EPSP synthases have a high tolerance to glyphosate.
Plant cells resistant to glyphosate toxicity can be produced by transforming plant cells to express glyphosate-resistant bacterial EPSP synthases. Notably, the bacterial gene from Agrobacterium tumefaciens strain CP4 has been used to confer herbicide resistance on plant cells following expression in plants. A mutated EPSP synthase from Salmonella typhimurium strain CT7 confers glyphosate resistance in bacterial cells, and confers glyphosate resistance on plant cells (U.S. Pat. Nos. 4,535,060; 4,769,061; and 5,094,945). However, there is a need for other herbicide resistance genes.
EPSPS kinetic activity can be assayed by measuring the liberation of phosphate. Phosphate liberation is detected using a coupled assay for the fluorescent detection of phosphate based on the generation of N-acetyl-3,7-dihydroxyphenoxacine (Amplex® Red), as is known in the art (Vazquez et al. (2003) Analytical Biochemistry 320: 292-298). The published assay conditions can lead to saturation of the assay in experiments where phosphate is liberated very quickly. Additional methods are needed for the measurement of EPSPS kinetic activity.